1. Field of Invention
The present invention relates to diamond-like carbon coating compositions, and more particularly, to metal-free diamond-like-carbon coating compositions and methods of making and applying such coatings.
2. Description of the Related Art
Diamond-like carbon (DLC) films or coatings are hard and have low friction coefficients (μ), especially against steel. Such films have numerous applications, especially in the fields of machine elements and tools.
DLC coatings, consisting of a highly cross-linked network of carbon atoms, typically have high compressive stress (typically several GPa). These high stress values lead to poor adhesion with the substrate, especially on steel and, therefore, limit their use in practical applications. The preparation and application of DLC coatings can be problematic. The most common method of preparation and application of DLC coatings is the use of radio frequency (r.f.) glow discharge of hydrocarbon gases with negatively self-biased substrates. However, the scaling up of this r.f. technique to industrially relevant dimensions, geometries and throughput requirements is often difficult, e.g., on the inner surface of journal sleeves.
Metal containing diamond-like carbon (Me-DLC) films or coatings have low metal content (atomic ratios of Me/C typically up to approximately 0.3), and have a lower compressive stress than a DLC coating (<1 GPa). The Me-DLC coatings consist of a network (continuous phase) of amorphous hydrogenated carbon (DLC) with incorporated or embedded metal carbides. The metal carbides typically have dimensions of several nanometers.
The friction coefficients of Me-DLC coatings are similar to those of DLC coatings. However, the abrasive wear resistance of Me-DLC coatings is generally thought to be lower than that of DLC coatings. At one time, the lowest abrasive wear rates reported for Me-DLC coatings were at least a factor of two lower than those reported for DLC coatings. Additionally, Me-DLC coatings typically require an intermediate adherence-promoting interlayer.
Different preparation and deposition techniques can be used for these coatings as discussed in an article entitled “Effect of target material on deposition and properties of metal-containing DLC (Me-DLC) coatings”, by K. Bewilogua, C. V. Cooper, C. Specht, J. Schroder, R. Wittorf and M. Grischke, Surface & Coatings Technology 127, 224-232, Elsevier (2000), which is incorporated by reference herein in its entirety. The results of DLC coatings with high hydrogen contents are discussed in an article entitled “DLC based coatings prepared by reactive d.c. magnetron sputtering”, by K. Bewilogua, R. Wittorf, H. Thomsen and M. Weber, Thin Solid Films, Vol. 447-448, pp. 142-147, 2004, which is incorporated by reference herein in its entirety.
Accordingly, there exists a need for a diamond-like-carbon coating composition having a superior hardness and low-residual stress. There is a further need for such coatings that possess high resistance to abrasion and superior adherence to substrates, e.g., metal substrates. There is an additional need for such coatings with improved resistance to failure in pure rolling compared to commercially available metal and hard diamond-like-carbon coating compositions.